The invention relates to a process and an apparatus for producing multilayers, which have on both of their respective outer sides an outer electrically conducting foil, in particular a metal foil, and between them at least one inner layer with strip conductors, wherein these foils and the inner layer(s) are connected to each other by intermediate layers made of an adherable dielectric and by the action of pressure and temperature, such that several multilayers are stacked on top of each other, while a separation layer is inserted between them, and are then simultaneously pressed, whereby before pressing the separation layer is supplied to the foil stack together with the upper metal foil of the multilayer lying underneath it and the lower metal foil of the multilayer lying above it, and the prior application of the metal foils occurs on both sides of the separation layer, optionally in a clean room.
Multilayers of this type are required for manufacturing electric multilayer switches. Many different inner layers can be located in them between the two outer metal foils, wherein these can optionally also have electronic components.
The two outer layers are generally made out of thin copper foils. Lying between them, depending on the structure, are one or more electrically conducting inner layers, which are connected to each other and to the two outer metal foils by so-called prepregs. These prepregs are intermediate layers impregnated with adhesive, preferably resin-impregnated glass fiber webs, in which the resin functioning as adhesive is partially polymerized to the extent that the prepregs are not yet adhesive at room temperature. However, the resin acting as adhesive and dielectric begins to melt at higher temperatures and cures completely under the action of temperature and pressure. The individual layers of the multilayer are then inseparably connected to each other, and the electrically conducting layers are isolated from each other.
So that the multilayer can be manufactured economically, it is customary to stack several multilayers on top of each other and then to compress them at the same time. With this mutual compressing, however, the outer metal foils would each rest on adjacent multilayers, which has the result that the metal foils would adapt to the resin flow of the prepreg melting below them and thus would no longer be smooth and even. However, it is necessary to have as even an outer metal layer as possible, because by etching off the unnecessary metal surface, the strip conductors of the outer switch can be generated from it. These strip conductors cannot be manufactured with the necessary degree of precision with an uneven outer metal layer.
It is therefore known, in order to keep the outer metal foils smooth, to insert a separation layer, in general a separation sheet, between the respective multilayers that are stacked on top of each other. This separation sheet has a very fine surface structure, so that the metal foils pressed against it are not deformed, but instead are kept planar and smooth.
In case prior to the compression of the foil, foreign substances, in particular dirt particles, come between the separation sheet and the metal foil, this would lead to the rejection of the multilayer, since the particles could punch through the metal foil and consequently cause an interruption in one of the strip conductors prepared later, and thus make necessary a later repair or lead to rejection.
Similarly, it is disadvantageous if these particles are made of resin, because the resin spreads over a large area when it melts and adheres to the separation sheet. In the metal foil not only would the already described irregularity then result, but also the separation sheet must then be expensively cleaned.
The prepegs themselves constitute a great danger of impurities, since they must first be cut to size and then stacked with the other layers on top of each other. In this condition, splinters from the glass fiber web or loosely adhering resin pieces can be released and fall on the metal foil or the separation sheet. Painstaking attention must thus be paid at the assembly point that the respective sides of the metal foils and the separation sheets, which come to lie against each other during the compression operation, are absolutely free of contaminants.
It is thus also known to handle the inner layers and prepegs separately from the separation sheet and the metal foil, particularly such that the metal foils and separation sheets are positioned and laid together in a clean room, while the inner layers and prepegs are positioned and laid together in a standard room. The metal foils are thereby adhered below and above to the separation sheet. The individual parts of the multilayer then go back and forth between standard room and clean room, since they are laid on at different locations. The production thereby becomes somewhat more complicated, and the transport between standard room and clean room generates an additional risk of contamination.
Starting from this background, an object of the invention is to improve the manufacture of multilayers going forward, so that the risk of contamination between the separation layerxe2x80x94designated above as a separation sheetxe2x80x94and the metal foil is for the most part eliminated. Moreover, the invention should distinguish itself by simple process steps.
This object is achieved according to the invention in that the metal foil is placed around the separation layer on its underside, end side and upper side in a U-shape, before the separation layer with the two-sided metal foil is supplied to the unfinished multilayer stack.
The metal foil thus practically encases the separation layer and protects it, as well as the intermediate space between it and the separation layer, from the environment. A penetration of contaminants, in particular during the transport of the separation layer and its metal foil, is greatly reduced in this way.
It is especially favorable when the metal foil is placed around the forward-lying end side of the separation layer in the travel direction of the multilayer structure, since on the forward side the danger of contamination is greater than it is in the back.
In practice, the desired encasing of the separation layer can be produced by working with a metal foil that is approximately twice as long as a separation layer, whereby the separation layer is then placed on the rear half of the metal foil in the travel direction of the multilayer structure, and the uncovered front half of the metal foil is finally folded over 180xc2x0 onto the upper side of the separation layer. For this purpose, a conveyor belt subdivided approximately in the middle is used, on which the metal foil is placed, so that then the separation layer is laid over it, and then an application roller arranged in the subdivision area of the conveyor belt grasps onto the metal foil from below, is pivoted upwardly, and finally the metal foil is rolled on top of the separation layer. With additional re-press rolling, a planar deposition of the metal foil on the separation layer is ensured and trapped air is rolled out.
With the encasing according to the invention, the additional advantage is offered that the separation layer can be covered with its metal foil in a clean room, and only after its encasing is it conveyed into the standard room and placed on the layers of the uppermost multilayer that are already stacked there on each other. The inner layers and prepegs can thus always remain in the standard room and not lead to any contamination of the clean room.
So that the transport of the separation layer with its metal foil from the clean room into the standard room is additionally protected from contamination, it is recommended to seal off the intermediate space between the separation layer and the metal foil also on the two opposite-lying sides running in the transport direction. This can preferably be done by grippers that carry out the transport at the same time. In this manner, the metal foil is constantly held on the separation layer without adhesive.
After the compression and curing operation, the multilayers stacked on top of each other are still connected to each other by the metal foils. The separation of the multilayers is possible at any time by a simple cutting of the metal foil in the transition area, and for this purpose the multilayers are expediently unfolded, and the separation sheet removed.